Inter-symbol interference (ISI) may arise when multiple instances of a transmitted signal are received at different times and with different signals strengths. The multiple instances may correspond to echoes or multi-path between a single transmitter and a receiver, and/or to multiple original instances of a signal, each transmitted from a corresponding transmitter, such as in a single frequency network (SFN).
ISI may impact channel equalization and demodulation. ISI may be reduced or eliminated in an OFDM system by adjusting a Fast Fourier Transfer (FFT) trigger point.
A signal may include a guard interval during which a FFT is to be triggered. When only one instance of a signal is received, the FFT may be triggered anywhere within the guard interval without incurring ISI. Where multiple instances of a signal are received within a delay spread that is less than the guard interval, there is a trigger point within the guard interval that will not result in ISI. Where the delay spread is greater than the guard interval, there will be ISI with respect to all points within the guard interval, which may be minimized with adjustment to the FFT trigger.
A FFT trigger point may be controlled by a symbol timing recovery process, which may include tracking a channel impulse response and adjusting the FFT trigger point in response to the channel impulse response. The channel impulse response may be determined from estimated channel frequency responses, such as with an inverse FFT (IFFT).
In a SFN, where transmitters are spaced relatively close to one another, a delay spread of signals from multiple transmitters may be similar to delay spreads associated with echoes and multi-paths in a single transmitter environment. Where transmitters are spaced relatively distant from one another, the delay spread may be relatively great.
SFNs are used in terrestrial digital video broadcast (DVB) to broadcast the same television signal at the same frequency from all transmitters in the network and thereby reduce the usage of the radio spectrum. In multi-frequency networks (MFNs) same television signal is transmitted at different frequencies by different transmitters to avoid interference at the receivers. In SFNs the interference from different transmitters are treated as echoes and are cancelled at the receiver using signal processing techniques.
Terrestrial DVB signals may be transmitted in accordance with one or more standards, such as a DVB-Terrestrial (DVB-T) standard and a DVB-Second Generation Terrestrial (DVB-T2) standard, promulgated by the Digital Video Broadcasting Project, an industry led consortium (http://www.dvb.org/index.xml), including standard, ETSI EN 302 755 V1.1.1, (2009-09), titled, “Digital Video Broadcasting (DVB); Frame Structure Channel Coding and Modulation for a Second Generation Digital Terrestrial Television Broadcasting System (DVB-T2),” publicly available at http://www.dvb.org/technology/dvbt2/. ETSI EN 302 755 V1.1.1 provides for a P2 symbol having L1-pre signaling, described below with respect FIG. 4.
An orthogonal frequency division multiplex (ODFM) signal may include periodic pilots on every mth carrier signal. The pilots may occur at different carrier frequencies in different symbols, and are thus referred to as scattered pilots. Time interpolation or time prediction may be used to estimate channel frequency responses of every nth carrier signal from the scattered pilots. For example, m may be equal to 12 and n may be equal to 3.
Where channel frequency responses are only available for every nth carrier frequency, an IFFT provides indistinguishable periodic replicas of the channel impulse response. This is referred to herein as channel impulse response cyclic time ambiguity.
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